Method and system for correcting an aberration of a beam of charged particles

ABSTRACT

A beam of charged particles is deflected in a closed path such as a square, for example, over a cross wire grid at a constant velocity by an X Y deflection system. A small high frequency jitter is added at both axes of deflection to cause oscillation of the beam at 45* to the X and Y axes. From the time that the leading edge of the oscillating beam passes over the wire until the trailing edge of the beam passes over the wire, an envelope of the oscillations produced by the jitter is obtained. A second envelope is obtained when the leading edge of the beam exits from being over the wire until the trailing edge of the beam ceases to be over the wire. Thus, a pair of envelopes is produced as the beam passes over each wire of the grid. The number of pulses exceeding ten per cent of the peak voltage in the eight envelopes produced by the beam completing a cycle in its closed path around the grid are counted and compared with those counted during the previous cycle of the beam moving in its closed path over the grid. As the number of pulses decreases, the quality of the focus of the beam increases so that correction signals are applied to the focus coil in accordance with whether the number of pulses is increasing or decreasing.

United States Patent 11 1 1 1 3,924,156 Doran et al. 1 Dec. 2, 1975METHOD AND SYSTEM FOR CORRECTING [57] ABSTRACT AN ABERRATION OF A BEAMOF A beam of charged particles is deflected in a closed CHARGEDPARTICLES path such as a square. for example, over a cross wireInventors: Samuel K. Doran, Wappingers Falls; grid at a constantvelocity by an X Y deflection sys- Merlyn H. Perkins, Hopewell tem. Asmall high frequency jitter is added at both Junction, both of NY. axesof deflection to cause oscillation of the beam at 45 to the X and Yaxes. From the time that the leading edge of the oscillating beam passesover the wire until the trailing edge of the beam passes over the [22]Filed: June 26, 1974 wire, an envelope of the oscillations produced bythe jitter is obtained. A second envelope is obtained when [21] Appl NO"483,266 the leading edge of the beam exits from being over the wireuntil the trailing edge of the beam ceases to be [73] Assignee:International Business Machines Corporation, Armonk, NY.

[52] US. Cl. i. 315/382 over the wire. Thus, a pair of envelopes isproduced as [51] Int. Cl. H01J 29/70 the beam passes over each wire ofthe grid. The num- [58] Field of Search .1 315/382, 30, 31, 364 ber ofpulses exceeding ten per cent of the peak voltage in the eight envelopesproduced by the beam com- [56] References Cited pleting a cycle in itsclosed path around the grid are UNITED STATES P TENTS counted andcompared with those counted during the 3,588,586 6/1971 Yanaka 315/379Previous cycle of the beam moving in its closed Pam 3,753,035 8/1973Verth 315/370 ever the grid- AS the "umber of PulseS deereeeee thequality of the focus of the beam increases so that corp Examiner MaynardWilbur rection signals are applied to the focus coil in accor- Assistampotenza dance with whether the number of pulses is increasing Attorney,Agent, or FirmFrank C. Leach, Jr.; or decreasing- Theodore E. Galanthay28 Claims, 16 Drawing Figures 772- OSCILLOSCOPE CURRENT 10 131111111115575 7e 89 97 9a 100 111 11? 42 VOLTAGE 1111111111111 f 7 f 2% CONVERTERAMPLIFIER 1u7o11111c l 11515011111 UP 1101111 FOCUS 11111 111110011111110 couurmc c011711o1 71 L12 150 00111101 11E111s 99/7 115115 T11111 couunzn 15s 1151mm;

7 1511s fm H I III voumzm 154 152 81111110 '6 7 D t 0 155 T 10m 153 5193 US. Patent Dec. 2, 1975 Sheet 1 of 6 3,924,156

9 1 Dn LL F MM P N M II D w c 8 7 1 n0 NO G WWW u m WU LN AN 00 U CC AUS. Patent Dec. 2, 1975 Sheet 2 of6 3,924 156 [m H4 [175 180 +5V181 1828-12BIT SQUARER COMPARATOR COUNTER g m (L COMPARATOR INVERTER 93 a BITDAC REF /185 FEG. 13 j m +5v iv 191 192 186 185 187 f NAND INVERTERSTATUS A COMPARATOR DECODER K INVERTER L 189 NUMBER OF PULSES FOCUS COILCURRENT Patant Dec. 2, 1975 Sheet 4 of 6 MULTIPLIER T k POSITIVE VOLTAGEif INVERTER COMPARATOR 97 L 9o 92 95 +01v NAND W 95 7 96 7 94 NEGATIVESINGLE VOLTAGE SHOT INVERTER COMPARATOR US. Patant Dec. 2, 1975 Sheetof6 324j5 101 98 1 FEG. 11

K 0 111 101;; FLIP 12 BIT UP 1 I C FLOP 0011111 COUNTER J 6 104 L F- 115SINGLE 3|NGLE $1101 1 119 W 1 INVERTER 11a 5 f 1115 111 156 S FLIP Q I L155 5STATE SINGLE A COUNTER $1101 151 121 g 1 11111110 127 126 m FLIP I8 BIT UP FLOP 124 DOWN COUNTER 129 8BIT 01c -1 128 METHOD AND SYSTEM FORCORRECTING AN ABERRATION OF A BEAM OF CHARGED PARTICLES In U.S. Pat. No.3,644,700 to Kruppa et al, there is shown an apparatus for stepping asquare-shaped beam of charged particles in a substantially rasterfashion over a predetermined area. An improved apparatus for stepping asquare-shaped beam of charged particles over a predetermined area isshown and described in the copending patent application of Michel S.Michail et al for Method And Apparatus For Positioning A Beam Of ChargedParticles, Ser. No. 437,585, filed Jan. 28, 1974, now U.S. Pat. No.3,900,736 and assigned to the same assignee as the assignee of thisapplication.

In the aforesaid Kruppa et al patent and the aforesaid Michail et alapplication, it is necessary for the beam to be properly focused toobtain the desired precise application of the beam in various portionsof the predetermined area. As described in the aforesaid Kruppa et alpatent and the aforesaid Michail et al application, the beam is movedthrough A, B, and C cycles.

The present invention provides a method and system for measuring thequality of the focus of the beam of charged particles at variousintervals and automatically correcting the focus. These measurements andthe corrections occur during certain of the C cycles.

The present invention also is capable of ascertaining the illuminationof the beam of charged particles, the quality of the focus of the beamof charged particles, and the astigmatism of the beam of chargedparticles so that manual correction of these aberrations can occur.These corrections would occur when the electron gun is set up to producethe beam.

The present invention measures the quality of the focus of the beam bydirecting the beam in a predetermined closed path across a target whichwill interrupt the beam of charged particles whenever the beam iscompletely engaging the target. Since the beam is deflected inorthogonal directions, the quality of the focus of the beam in both ofits deflection directions is ascertained.

The present invention preferably accomplishes this by utilizing a crosswire grid as the target and moving the beam in a square over the grid ata constant velocity. Thus, the beam will pass over a portion of the wiregrid extending in the X direction, for example, next over a portion ofthe wire grid extending in the Y direction, then over another portion ofthe wire grid extending in the X direction, and finally over anotherportion of the wire grid extending in the Y direction during each cycleof movement of the beam over the grid.

By applying a high frequency jitter to oscillate the beam in a singledirection at an angle to each of X and Y axes, the oscillations producedby the jitter form two envelopes as the beam passes over a wire. A firstenvelope of the oscillations occurs from the time that the leading edgeof the beam engages the wire until the trailing edge of the beam isdisposed over the wire. A second envelope occurs when the leading edgeof the beam exits from the wire until the trailing edge of the beamexits from the wire.

Each of the envelopes is indicative of the current density of the beamin the direction in which the beam is being deflected. The envelopesrepresents a voltage proportional to the jitter component of the currentof the current collector. Since the jitter is sweeping a constant twopercent slice of beam area onto and off of the wire, the peak amplitudeof the envelope voltage is proportional to the average current densityof the two percent slice. Thus, each of the two envelopes produced bythe beam crossing a wire extending in the Y direction when the beam isdeflected in the X direction is a profile of the current densitydistribution of the beam in the X direction.

If the beam were free of aberrations, each of these 0 envelopes would besquare shaped. However, when the beam is out of focus, the edgedefinition of the beam is smeared so that the current density profilealso is smeared. Additionally, if the beam is astigmatic, the envelopesor profiles of the current density in one direction may be sharp whilethe profiles in the other direction may be smeared or vice versa.Furthermore, if the beam is not properly aligned so that it does nothave uniform illumination, the top of the current density profile willnot be flat but will be slanted.

Therefore, with the profile of the current density not being squareshaped when the beam has an aberration with the beam being squareshaped, the quality of the current density of the beam can beascertained by the use of the envelopes. The envelopes indicate whetherthe beam is properly focused, has no astigmatism, and is illuminatedproperly.

Accordingly, as the focus becomes worse, the sides of the profile oftheenvelope tend to become more inclined or curved. Since the areaunderneath the envelope must always be the same irrespective of whetherthe beam has aberrations or not, an indication of the quality of thefocus of the beam in comparison with the focus of the beam during theprior scan can be ascertained through determining whether the envelopesides are becoming more straight to indicate increasing quality of thefocus of the beam or more inclined to indicate decreasing quality of thefocus of the beam. If the beam has astigmatism, this will affect thefocus quality.

Similarly, the illumination of the beam also can be ascertained throughdetermining whether the envelope has a flat top or not. If the topceases to be flat, then the area near the top of the envelope tends tobecome smaller to indicate poor illumination.

Accordingly, the quality of the focus of a beam can be compared to thatduring the previous pass of the beam over the wire by counting thenumber of times that the voltage within the. envelope exceeds a certainlimit, which is preferably ten percent of the peak voltage produced bythe jitter. At ten percent of the peak voltage produced by the jitter,the inclination of the sides of the profile of the envelope is readilycomparable with the inclination of the sides of the profiles of theenvelope produced during the prior pass of the beam over the wire. Ofcourse, other suitable percentages, except fifty percent, of the peakvoltage could be employed.

When initially starting to measure the quality of the focus, thecomparison is with the information from the satisfactory focus of thebeam from the previous C cycle in which focusing occurred. It should beunderstood that other operations concerning the quality of the beamoccur during other C cycles so that a number of A, B, and C cyclesnormally occur between focus measurements and noise could haveeliminated the prior information. Thus, the signal produced by the firstcycle of movement of the beam over the target could produce a correctionsignal in the wrong direction.

If the number of times the voltage exceeds the selected limit increasesbeyond the number from the prior cycle, than the direction of change ofthe magnitude of current to the focus coil is reversed. If the count ofthe pulses is less than that produced by the prior pass of the beam overthe grid, the direction of change of the magnitude of current to thefocus coil is continued.

There are a total of sixteen complete cycles of the beam along itsclosed path over the cross wire grid during each C cycle in whichautomatic focus correction is occurring. This is sufficient tosatisfactorily focus the beam. Whenever the direction of change of themagnitude of the current is reversed three consecutive times when thecorrection signal is supplied to the focus coil, the best focus valuehas been obtained.

To ascertain whether the illumination of the beam is satisfactory, thecounting of the number of times that the voltages within the envelopeexceeds another voltage limit, which is preferably ninety percent of thepeak voltage produced by the jitter, enables ascertainment of thequality of the illumination of the beam. Thus, when the number of timesthat the voltage within the envelope exceeds the selected limitincreases from the prior pass of the beam over the grid, this is anindication that the illumination of the beam is being improved. This isbecause an increase in the number of times that the voltage exceedsninety percent of the maximum voltage produced by the jitter is anindication of an increase in area in the top portion of the profile sothat the top of the profile of the envelope is becoming more flat.

An object of this invention is to provide a method and system forcorrecting an aberration of a beam of charged particles.

Another object of this invention is to provide a method and system forfocusing a beam of charged particles.

A further object of this invention is to measure the quality of thefocus of the beam of charged particles and adjust the focus inaccordance with the quality.

Still another object of this invention is to automatically correct thefocus of a beam of charged particles at selected intervals of time.

The foregoing and other objects, features, and advantages of theinvention will be more apparent from the following more particulardescription of the preferred embodiment of the invention as illustratedin the accompanying drawings.

In the drawings:

FIG. 1 is a schematic view showing a beam of charged particles and theapparatus for controlling the beam.

FIG. 2 is a schematic top plan view ofa portion of the wire grid overwhich the beam of charged particles moves during focusing.

FIG. 3 is a schematic plan view of the beam showing the beam in itsposition when not moved by the jitter frequency and the position towhich it is moved by the jitter frequency.

FIG. 4 is a schematic view showing a pulse produced by the beam passingover a portion of the wire grid with the oscillations from the jitterfrequency appearing on the pulse as the beam enters and leaves theportion of the wire grid.

FIG. 5 is a view showing two different shaped envelopes produced by theoscillations from the high frequency jitter resulting from the beamentering or leav- 4 ing a portion of the wire grid with one of theenvelopes representing a good focus condition of the beam and the otherof the envelopes representing a poor focus condition of the beam.

FIG. 6 is a view of the shape of an envelope produced by theoscillations from the high frequency jitter resulting from the beamentering or leaving a portion of the wire grid when illumination of thebeam is poor.

FIG. 7 is a block diagram showing the relationship of circuitry used forcorrecting aberrations of the beam.

FIG. 8 is a schematic wiring diagram of the bandpass filter andamplifier of the circuitry of FIG. 7.

FIG. 9 is a schematic wiring diagram of the automatic gain control ofthe circuitry of FIG. 7.

FIG. 10 is a block diagram of the detecting and counting means of thecircuitry of FIG. 7.

FIG. 11 is a block diagram of an up down counting means of the circuitryof FIG. 7.

FIG. 12 is a block diagram of the focus control unit of the circuitry ofFIG. 7.

FIG. 13 is a block diagram of an illumination detecting means of thecircuitry of FIG. 7.

FIG. 14 is a block diagram of the apparatus for producing the jitterfrequency.

FIG. 15 is a timing diagram showing the relationship of various signalsduring focusing of the beam.

FIG. 16 is a schematic diagram showing the relationship of the focuscoil current and the number of pulses counted during cycles of the beampassing over the wire grid.

Referring to the drawings and particularly FIG. 1, there is shown anelectron gun 10 for producing a beam 11 of charged particles in thewell-known manner. As shown and described in the aforesaid Michail et alapplication, the electron beam 11 is passed through an aperture 12 in aplate 14 to shape the beam 11. The beam 11 is preferably square shapedand has a size equal to the minimum line width of the' pattern that isto be formed.

The beam 11 passes between a pair of blanking plates 15, which determinewhen the beam 11 is applied to the material and when the beam. 11 isblanked. The blanking plates 15 are controlled by circuits of an analogunit 17, which has a column control unit 16 connected thereto.

The analog unit 17 is controlled by a digital control unit 18 in themanner more particularly shown and described in the copending patentapplication of Phillip M. Ryan for Method And Apparatus For ControllingMovable Means Such As An Electron Beam, Ser. No. 398,734, filed Sept.19, 1973, now U.S. Pat. No. 3,866,013, and assigned to the same assigneeas the assignee of this application. The digital control unit 18 isconnected to a computer 19, which is preferably an IBM 370 computer.

The beam 11 then passes through a circular aperture 20 in a plate 21.This controls the beam 11 so that only the charged particles passingthrough the centers of the lenses (not shown) are used so that asquare-shaped spot without any distortion is produced.

The beam 11 is next directed through stigmator coils 21A and 21B andthen through a focus coil 22. The stigmator coils 21A and 21B and thefocus coil 22 are connected to the column control unit 16.

After passing through the focus coil 22, the beam is directed throughmagnetic deflection coils 23, 24, 25, and 26. The magnetic deflectioncoils 23 and 24 control the deflection of the beam 11 in a horizontal orX direction while the magnetic deflection coils 25 and 26 control thedeflection of the beam 11 in a vertical or Y direction. Accordingly, thecoils 23-26 cooperate to move the beam 11 in a horizontal scan byappropriately deflecting the beam 11.

While the beam 11 could be moved in a substantially raster fashion asshown and described in the aforesaid Kruppa et al patent, it ispreferably moved in a back and forth scan so that the beam 11 moves inopposite directions along adjacent lines as shown and described in theaforesaid Ryan and Michail et a]. applications. Thus, the negativebucking sawtooth of the type shown in FIG. 3b of the aforesaid Kruppa etal patent is supplied to the coils 23 and 24 during forward scan while apositive bucking sawtooth, which is of opposite polarity to the sawtoothshown in FIG. 3b of the aforesaid Kruppa et al patent, is supplied tothe coils 23 and 24 during the backward scan.

As shown and described in the aforesaid Michail et al application, thebeam 11 then passes between a first set of electrostatic deflectionplates 27, 28, 29, and 30. The electrostatic deflection plates 27 and 28cooperate to deflect the beam in a horizontal or X direction while theelectrostatic deflection plates 29 and 30 cooperate to move the beam 11in the vertical or Y direction. The plates 27-30 are employed to provideany desired offset of the beam 11 at each of the predetermined positionsor spots to which it is moved. In the aforesaid Kruppa et al patent, theplates 27-30 corrected for linearity, but these correction signals aresupplied to the coils 23-26 in the aforesaid Michail et al applicationand in this application.

After passing between the electrostatic deflection plates 27-30, thebeam 11 then passes between a second set of electrostatic deflectionplates 31, 32, 33, and 34. The electrostatic deflection plates 31 and 32cooperate to deflect the beam 11 in the horizontal or X direction whilethe electrostatic deflection plates 33 and 34 cooperate to deflect thebeam 11 in the vertical or y direction. The plates 31 and 32 deflect thebeam 11 in the X direction and the plates 33 and 34 deflect the beam 11in the Y direction from each of the predetermined positions to which itis moved in accordance with its predetermined pattern so that the beam11 is applied to its actual position based on the deviation of the areafrom its designed position, both shape and location, in which the beam11 is to write as more particularly shown and described in the aforesaidMichail et al application.

The beam 11 is then applied to a target, which is supported on a table35 and can be a semiconductor wafer, for example. The table 35 ismovable in the X and Y directions as more particularly shown anddescribed in the aforesaid Kruppa et al patent.

The beam 11 is moved through A, B, and C cycles as shown and describedin the aforesaid Kruppa et al patent and the aforesaid Michail et alapplication. The present invention is particularly concerned withsupplying signals to automatically correct the focus of the beam 11during some of the C cycles.

The focus of the beam 11 is controlled by the focus coil 22. A currentflows through the focus coil 22 to regulate the focus of the beam 11.

When a program in the computer 19 determines that it is time for thefocus of the beam 11 to be automatically corrected, if necessary, duringa C cycle, the table 35 is moved to bring a cross wire grid 41 (see FIG.2), which is in the same plane as the semiconductor wafer 6 supported onthe table 35, beneath the undeflected beam 11. A current collector 42(see FIG. .7), which is preferably a photodiode, is disposed beneath thegrid 41, which is preferably formed of. tungsten and comprises aplurality of orthogonal wires extending in the X and Y directions.

After the grid 41 has been properly positioned beneath the beam 11, thedigital control unit 18 causes a FOCUS SERVO signal, which is a positivepulse as shown in FIG. 15, to be supplied from the analog unit 17. Theanalog unit 17 suplies the FOCUS SERVO signal over a line 43 (see FIG.14) and through an optical isolator 44 to an OR gate 45. The opticalisolator 44 has a return line 45 to ground. One-suitable example of theotical isolator 44 is sold by Hewlett-Packard as model 5082-4350.

The OR gate 45 has its output connected as an input to a NAND gate 46.The other input to the NAND gate 46 is from a 300 KHZ oscillator 47.

When the FOCUS SERVO pluse is supplied to the optical isolator 44, theNAND gate 46 allows the oscillator 47 to supply its frequency through adither control 48, which controls the amplitude of the high frequencyjitter supplied through a line 49 to the analog unit 17 to each of theX'and Y electrostatic deflection plates 31, 32, 33, and 34 (see FIG. 1The jitter is preferably two percent of the width of the beam 11. Sinceit is applied to both the X and Y electrostatic deflection plates 31-34in phase, the jitter is at an angle of 45 with respect to the X and Yaxes so that the beam 11 oscillates in a single direction. Thus, in FIG.3, the beam 11 is shown in solid lines in its position when jitter isnot applied and in phantom lines when jitter is applied.

The current, which is produced from the current collector 42, has ajitter component due to the high frequency jitter. This jitter componentis equal to the product of the current density of the beam 11 and thearea of the jitter of the beam 11.

When the FOCUS SERVO signal is supplied to the optical isolator 44, thebeam 11 is caused to move along a square-shaped path 60 (see FIG. 2) soas to cross portions 61, 62, 63, and 64 of the wire grid 41 in thisorder. Thus, the beam 11 is moved in a predetermined closed path aroundthe wire grid 41.

The beam 11 is initially moved in a +X direction as indicated by anarrow 65 so as to cross the vertical portion 61 of the wire grid 41during this scan. This initial movement of the beam 11 is caused by a +XSCAN signal being supplied from the analog unit 17 due to the digitalcontrol unit 18 supplying a digital signal thereto. This +X SCAN pulsestarts at the same time that the FOCUS SERVO signal begins as shown inFIG. 15.

Thus, as shown in thetiming diagram of FIG. 15, an increasing X magneticdeflection voltage is initially supplied to the X magnetic deflectioncoils 23 and 24 to move the beam 11 in the +X direction. At the end ofthe increase in the X magnetic deflection voltage, a decreasing Ymagnetic deflection voltage is supplied to the Y magnetic deflectioncoils 25 and 26 so that the beam 11 moves in a Y direction as indicatedby an arrow 66 in FIG.- 2 to pass over the horizontal portion 62 of thewire grid 41. When the Y magnetic deflection voltageceases to fall tostop movement in the Y direction, a decreasing X magnetic deflectionvoltage is supplied to the X magnetic deflection coils 23 and 24 to movethe beam 11 in a X direction as indicated by an arrow 67 inFIG. 2 topass over the vertical portion 63 of the wire grid 41. When the Xmagnetic deflection voltage stops falling so that movement in the Xdirection stops, an increasing Y magnetic deflection voltage is suppliedto the Y magnetic deflection coils 25 and 26 to move the beam 11 in a +Ydirection as indicated by an arrow 68 in FIG. 2 to pass over thehorizontal portion 64 of the wire grid 41. As a result, the beam 11 ismoved along the square-shaped predetermined path 60.

As shown in FIG. 4, the output of the current collector 42 is at amaximum when the beam 1 1 is completely off the wire grid 41, decreasesfrom its maximum to its minimum as the beam 11 moves over the wire grid41, is at a minimum when the beam 11 is completely disposed above one ofthe portions 61-64 of the wire grid 41, and increases from its minimumto its maximum as the beam 11 moves off of one of the portions 61-64 ofthe wire grid 41.

During the time that the beam 11 is moving onto or off of one of theportions 61-64 of the wire grid 41, the current at the current collector42 includes the oscillations produced by a high frequency jitter fromthe dither control 48. The high frequency jitter produced by the beammoving onto one of the portions 61-64 of the wire grid 41 is indicatedon the pulse of FIG. 4 by 68A and the high frequency jitter produced bythe beam 11 moving off of one of the portions 61-64 of the wire grid 41is indicated on the pulse of FIG. 4 by 68B. This high frequency jitterproduces an envelope which is indicative of the quality of the focus ofthe beam 11.

Two different envelopes 69 and 70 of the high frequency jitter are shownin FIG. 5. Each of the envelopes 69 and 70 has the same area thereundersince the beam 11 produces the same current.

If the beam 11 were free of aberrations, it would have a square-shapedenvelope since the current density would be the same in all parts of thebeam 11. However, when the beam 11 goes out of focus, the edgedefinitions thereof become smeared so that the current density profile,which is represented by each of the envelopes 69 and 70, for example, ofthe beam 11 becomes smeared. If the beam 11 is astigmatic, the envelopeproduced by the beam 11 moving in one of the X and Y scans may be sharpbut movement of the beam 1 1 in the other scan direction would produce asmeared envelope.

It should be understood that each of the envelopes 69 and 70 is formedby voltages which are directly related to the jitter component of thecurrent from the current collector 42. This is because the output of thecurrent collector 42 is supplied to a current to voltage converter 71(see FIG. 7), which can be an operational amplifier, for example, andthe envelopes 69 and 70 represent the output of the voltage converter71, which supplies an inverted output.

By comparing the number of times that the voltage reaches some level ineach of the envelopes 69 and 70, a determination can be made as to whichof the two envelopes 69 and 70 produces the best focus. Thus, forexample, if a count is made each time that the voltage in the envelope69 or 70 exceeds ten percent of the peak voltage, a comparison can bemade as to which of the two envelopes 69 and 70 is producing the betterfocus since the envelope having the lowest number of times that thevoltage exceeds ten percent of the maximum is a better envelope forfocus.

A total of eight envelopes is produced during each movement of the beam11 along the entire path 60 since an envelope is produced each time thatthe beam 11 moves on or off of the wire grid 41. By comparing the totalnumber of times that the voltage exceeds ten percent of the peak voltagein two consecutive movements of the beam 1 1 around the wire grid 41along the path 60, a comparison between the two cycles of movement ofthe beam 11 along the entire path 60 is obtained so that it can beascertained whether a correction signal to the focus coil 22 in the timebetween the two cycles improved or lowered the quality of the focus ofthe beam 11.

When the beam 11 starts to cross the vertical portion 61 of the wiregrid 41, for example, the current collector 42 produces a varyingcurrent with the dither thereon. The signal from the current collector42 is supplied to the current to voltage converter 71, which supplies aninverted output voltage to a bandpass filter and amplifier 72 (see FIG.7). The bandpass filter and amplifier 72 allows only the envelopevoltage, which is produced when the beam 11 is crossingone of theportions 61-64 of the wire grid 41, to pass therethrough to form theenvelopes.

As shown in FIG. 8, the bandpass filter and amplifier 72 includes a pairof differential amplifiers 73 and 74 cooperating to produce a Q of tenand a gain of 100 at 300 KHz. The output of the bandpass filter andamplifier 72 is supplied over a line 75 to an automatic gain control 76(see FIG. 7). The output of the bandpass filter and amplifier 72 also issupplied over a line 77 to an oscilloscope 78 located within the analogunit 17. The oscilloscope 78 enables any of the eight envelopes producedby movement of the beam 11 over the wire grid 41 to be viewed.

The automatic gain control 76 maintains the amplitude of the envelope atfourteen volts peak to peak to compensate for slow drifts in thebrightness of the beam 1 1. Thus, the maximum peak to peak voltageproduced on an output line 79 of the automatic gain control 76 isfourteen volts.

The focus envelope wave form can be described as a function of time byan expression of the form envelope V,,E(t)sin 2IIft where sin 2"represents a unity amplitude sine wave of frequencyf. E(t) is a voltagethat is a function of time that follows the positive peaks of theenvelope and is normalized to a maximum peak amplitude of one, and V isthe actual positive peak amplitude of the envelope.

As shown in FIG. 9, the automatic gain control 76 has the line '/5connected to lines 80 and 81. The line 80 supplies one of the two inputsto a multiplier 82. The line 81 is connected to a negative peak and holdcircuit through a diode 83. The output of the negative peak and holdcircuit at the output of a differential amplifier 84 is the negativepeak voltage, V,,. This output is supplied to a divider 85, whichreceives a constant input voltage on a line 86. The divider 85 invertsthe negative output of the differential amplifier 84 and converts thesignal on its output line 87 to a positive output of lOC/v, where C isthe constant voltage input on the line 86 and V, is the positive peakvoltage.

The multiplier 82 multiplies the input on the line 80 from the bandpassfilter and amplifier 72 and the input from the divider 85 on the line 87to supply an output voltage on the line 79- The output voltage on theline 79 is the envelope produced by the beam 1 l entering or leaving oneof the portions 61-64 of the wire grid 41 and is one-tenth of theproduct of the two multiplicands 10C/V,, and V,,E(t) sin Zn-ft; thus,the output voltage on the line 79 is C[E(t) sin 21rft]. This means thateach 9 of the focus envelopes at the line 79 has a constant amplitudedetermined by C while its shape, determined by E(r) sin 2IIit, is notdistorted.

The output voltage of the automatic gain control 76 is connected throughthe output line 79 and a line 88 (see FIG. 7) to a detecting andcounting means 89. The detecting and counting means 89 includescomparators 90 and 91 (see FIG. with each of the comparators producingan output when its threshold signal is crossed.

The threshold signal for the positive comparator 90 is ten percent ofthe positive peak voltage on the output line 79. Since the positive peakvoltage is 7 volts, the positive comparator 90 produces an output signaleach time that the voltage within the envelope exceeds 0.7 volt.Similarly, the negative comparator 91 produces an output each time thatthe voltage is more negative than ten percent of the negative peakvoltage. Thus, each time that the negative voltage within the envelopeexceeds ().7 volt, the comparator 91 produces an output pulse.

The output of the positive comparator 90 is connected to a single shot92 which produces a positive pulse whenever the comparator 90 producesan output, which is a positive pulse. The positive output of the singleshot 92 is inverted by an inverter 93 and supplied as a negative inputpulse to a NAND gate 94.

Similarly, the output of the negative comparator 91 causes a single shot95 to have a positive output pulse whenever the negative comparator 91has its threshold voltage crossed to produce a positive output. Thepositive pulse of the single shot 95 is inverted by an inverter 96 andsupplied as a negative input pulse to the NAND gate 94.

Accordingly, whenever either of the comparators 90 and 91 has itsthreshold voltage crossed to supply a negative input to the NAND gate94, a positive pulse occurs at the output of the NAND gate 94 since itproduces a negative pulse only when both of its inputs are positive.Thus, each output pulse from the comparators 90 and 91 appears as apositive pulse at the output of the NAND gate 94. Therefore, all of thepositive output pulses at the NAND gate 94 are a total of the number oftimes that the voltage within the envelope exceeds ten percent of thepeak positive or negative voltage when the beam 11 enters or leaves oneof the portions 61-64 of the wire grid 41.

The output of the NAND gate 94 is supplied through a line 97 to inputlines 98 and 99 (see FIG. 7) of an up down counting means 100. The inputline 98 supplies one of the inputs to a NAND gate 101 (see FIG. 11),which has its other input supplied from Q output of a clocked JK flipflop 102. The Q output of the flip flop 102 is tied to K input of theflip flop 102.

Thus, when the flip flop 102 is supplying a positive signal at the Qoutput, the NAND gate 101 produces a negative pulse on its output line103 each time that a positive pulse appears on the input line 98 to theNAND gate 101 from the output of the NAND gate 94. Accordingly, eachpulse counted by the NAND gate 94 from each of the comparators 90 and 91is supplied on the output line 103 when the Q output of the flip flop102 is positive.

The input line 99 of the up down counting means 100 comprises one of theinputs to a NAND gate 104, which has its other input supplied from Qoutput of the flip flop 102. The Q output is tied to J input of the flipflop 102.

Thus, when the Q output of the flip flop 102 is positive, the NAND gate104 produces a negative pulse on its output line 105 each time that theNAND gate 94 has a positive pulse on the output line 97. Therefore, ifthe flip flop 102 is set in the state in which the Q output is positive,then the NAND gate 104 counts the pulses from the NAND gate 94 andsupplies these on the line 105.

The flip flop 102 has its clock input C connected by a line 106 to asingle shot 107 (see FIG: 7). The single shot 107 is connected through aline 108 to receive the +X SCAN signal from the analog unit 17 (see FIG.1). Thus, each time that the beam 11 starts to move along the path 60(see FIG. 2), the single shot 107 (see FIG. 7) is fired to supply apulse to the C input of the flip flop 102 (see FIG. 11).

The negative going portion of the pulse from the single shot 107 tripsthe flip flop 102 to change its state. Therefore, each time that thebeam 11 is to start movement along the path 60, the state of the flipflop 102 is changed to cause the opposite of the NAND gates 101 and 104from that in the prior cycle of movement of the beam 11 along the path60 to be responsive to the positive pulses on the line 97.

The lines 103 and 105 are connected to a twelve bit up down counter 110.When the NAND gate 101 is responsive to the positive pulses from theNAND gate 94 due to the Q output of the flip flop 102 being positive,the counter 110 counts up. When the NAND gate 104 is responsive to thepositive pulses from the NAND gate 94 due to the Q output of the flipflop 102 being positive, the counter 110 counts down.

If the down count is greater than the up count, the counter 110 suppliesa negative pulse on its output line 111. No signal is supplied on theoutput line 111 if the down count does not exceed the up count. Onesuitable example of the counter 110 is sold by Fairchild Semiconductoras model 9366.

When the flip flop 102 has its state changed so that the NAND gate 101causes the counter 110 to count up, the signal from the Q output of theflip flop 102 is supplied not only to the NAND gate 101 but also to asingle shot 112, which produces a positive pulse on its output line 113when the Q output of the flip flop 102 goes positive. The output line113 is connected to the master reset of the counter 110 to reset thecounter 110 to zero prior to counting up.

When the flip flop 102 changes state to cause count down in the counter110, the Q output, which enables the NAND gate 104 to be activated byany positive pulse on the line 99, also causes a single shot 114 toproduce a positive pulse on its output line 1 15. The single shot 11 4produces the positive pulse when the Q output of the flip flop 102 goespositive to enable the NAND gate 104 for the pulses from the NAND gate94 to cause down count in the counter 110.

The output line 111 of the counter 110 is connected as one input to aNAND gate 116 (see FIG. 12) of a focus control unit 117. The other inputto the NAND gate 116 is from a flip flop 118, which is the same type asthe flip flop 102. The flip flop 118 receives a signal from the analogunit 17 at the start of the FOCUS SERVO signal to change its state. Thepulse, which is identified as FOCUS SERVO INITIATE in the timing diagramof FIG. 15, is applied through a line 119 from the analog unit 17. Aninverter 120 causes a negative pulse to be supplied to input S of theflip flop 118 whereby output Q of the flip flop 118 goes positive whenthe FOCUS SERVO INITIATE pulse is supplied on the line 119. Thus, apositive input is supplied to the NAND gate 116 from the Q output of theflip flop 118 at the start of the cycle for automatically focusing thebeam 11.

The Q output of the flip flop 118 also is connected as one input to aNAND gate 121, which receives its other input from the output line 115of the single shot 114. Thus, after the up count is completed and downcount is about to start so that the single shot 114 provides a positivepulse, the NAND gate 121 has its output change state to supply anegative pulse on its output line 122.

The line 122 is connected as one of the inputs to each of NAND gates 123and 124. The other input to the NAND gate 123 is from a flip flop 125,which is the same type as the flip flop 102. Thus, the NAND gate 123 isconnected to output Q of the flip flop 125. The other input to the NANDgate 124 is from output O of the flip flop 125.

The flip flop 125 has its clock input C connected by a line 126 to theoutput of the NAND gate 116. Thus, each time that there is a positivepulse from the NAND gate 116, the flip flop 125 changes state to causethe other of the NAND gates 123 and 124 to have its output responsive tosignals supplied from the NAND gate 121.

Each time that the NAND gate 121 provides a negative pulse on the outputline 122, the counter 110 has counted up. Thus, at the completion ofeach up count, which is a counting of all the times that the voltageexceeds ten percent of the peak voltage in the eight envelopes producedduring movement of the beam 11 one time around the path 60, one of theNAND gates 123 and 124, depending on the state of the flip flop 125,produces a pulse to an eight bit up down counter 127. One suitableexample of the eight bit up down counter 127 is sold by FairchildSemiconductor as model 9366.

The output of the counter 127 is connected to an eight bit digital toanalog converter (DAC) 128. One suitable example of the DAC 128 is soldby Beckman Instruments as model 845. The DAC 128 supplies a voltagethrough its output line 129 and a'coil driver 130 (see FIG. 7) to thefocus coil 22.

If the counter 110 (see FIG. 11) produces a signal on the output line111 at the completion of the down count, the NAND gate 116 (see FIG. 12)provides a positive pulse on the output line 126 to change the state ofthe flip flop 125. As a result, the other of the NAND gates 123 and 124produces an output in response to the output of the NAND gate 121 at thecompletion of the next cycle of the beam 11 along the path 60.

Each of the positive output pulses from the NAND gate 123 causes an upcount in the counter 127. Each of the positive output pulses from theNAND gate 124 produces a down count in the counter 127. Thus, if thepulse supplied to the counter 127 is from the NAND gate 123, the voltagefrom the eight bit DAC 128 in creases to cause an increase in thecurrent to the focus coil 22. If the NAND gate 124 supplies the pulse tothe counter 127, then the eight bit DAC 128 decreases the output voltageon the line 129 to cause the current to the focus coil 22 to decrease.

The counter 110 (see FIG. 11) produces a negative pulse on the outputline 111 only when the number of pulses in the down count exceeds thatduring the up count. This means that a worse focus exists during thesecond scan, which is when the down count is occur- 12 ring, thanexisted prior to change in the current in the focus coil 22 at the endof the first scan (an up count). Therefore, it is necessary to changethe direction in which the counter 127 (see FIG. 12) is being stepped orcounted so that the direction of change of the current to the focus coil22 is reversed.

Thus, when the next scan occurs after the down count so that the counteris counting up, the output on the line 122 of the NAND gate 121 at thecompletion of the up count would now be supplied to the opposite of theNAND gates 123 and 124 to shift the direction in which the counter 127counts. This reverses the direction of change of the current to thefocus coil 22. That is, if the current had been increasing, it will bedecreased. Similarly, if the current had been decreasing, it will beincreased.

Of course, if there is no output on the line 111 at the completion ofthe down count of the counter 110, then this means that the focus hasbeen improved by the change in the current to,the focus coil 22 at theend of the prior scan (up count). Therefore, the state of the flip flop125 is not altered, and the counter 127 continues to count in the samedirection.

Since it is desired for a minimum number of the pulses exceeding tenpercent of the negative or positive peak voltage to exist duringmovement of the beam 11 through a cycle along its path 60 to indicatethe best quality of the focus of the beam 1 1, there is a focus coilcurrent at which the number of pulses will be at a minimum. Referring toFIG. 16, there is shown a relationship between the number of pulses andthe current in the focus coil 22. It will be assumed that the number ofpulses is being reduced by each increase in the focus coil current atthe end of each up count by the counter 110 (see FIG. 11) with thecounter 127 (see FIG. 12) counting up because of pulses from the NANDgate 123 to increase the voltage from the DAC 128.

When the minimum number of pulses occurs, the next scan (a down count)will result in the number of pulses increasing as indicated by step 131in FIG. 16. When this occurs, the counter 110 (see FIG. 11) produces anegative pulse on the output line 111 because the number of pulsescounted during the down count exceeds those counted during the up countwhereby the flip flop 125 (see FIG. 12) has its state changed. As aresult, the current to the focus coil 22 will be decreased by the DAC128 producing a lower voltage since the counter 127 is now counted downat the end of the up count in the counter 110 by the NAND gate 124.

With the focus coil current now decreased during the next down count inthe counter 110, the number of pulses will be a minimum during thiscycle of movement of the beam 11 along the path 60. This is during adown count in the counter 110. Thus, there is no change in the state ofthe flip flop 125 at the end of the down count in the counter 110.

As a result, the current to the focus coil 22 is decreased at the end ofthescan producing the next up count in the counter 110. This increasesthe number of pulses beyond the minimum during the next down count inthe counter 110 so that the counter 110 again produces a negative pulseon the output line 111 at the end of the down count in the counter 110.This increase in the number of the pulses during the down count isindicated by step 132 in FIG. 16.

As a result of the negative pulse on the output line 111, the flip flop125 again changes state. This causes 13 the current to the focus coil 22to again be increased until the step 131 is again reached. At this time,the counter 110 again produces a pulse on the line 111 to again shiftthe state of the flip flop 125.

When three of these pulses have occurred on the line 111, the minimumnumber of pulses within the envelope will have been produced since thesethree changes indicate that the valley of the number of pulses has beenreached as indicated by curved line 133 in FIG. 16. When this occurs, itis desired to stop the focus control unit from functioning.

Accordingly, the output line 126 of the NAND gate 116 is not onlyconnected to the flip flop 125 but also to a three state counter 135(see FIG. 12). Thus, the counter 135 counts each time that the flip flop125 changes state. The counter 135 is actually a four bit counter withits third output being employed to activate a single shot 136. Onesuitable example of the counter 135 is sold by Fairchild Semiconductoras model 9366.

The single shot 136 produces a negative output pulse on its line 137 toreset input R of the flip flop 118.. This causes the Q output of theflip flop 118 to become negative whereby the NAND gate 121 can no longerchange state when a positive pulse is supplied from the line 115.Therefore, when the flip flop 125 has changed state three times so as toindicate that the minimum number of pulses are within the eightenvelopes, the counter 135 prevents further activation of the NAND gate121 so that there is no further change to the DAC 128. The DAC 128 holdsits final output until the next time a focus servo cycle is activated tosupply the FOCUS SERVO INITIATE pulse to the flip flop 118.

The single shot 136 also provides the negative pulse on its output line137 by a line 138 to the master reset of the counter 135. This negativepulse sets the counter 135 to zero.

As previously mentioned, there are eight envelopes produced by the beam11 crossing the four portions 61-64 (see FIG. 2) of the wire grid 41.The total times that the voltage in these eight envelopes exceeds tenpercent of the positive or negative peak voltage is what is counted inthe counter 110.

Furthermore, the beam 1 1 passes completely around the wire grid 41sixteen times. Even though two of these cycles are required for the upand down count to the counter 110, the eight times that there can becontrol of the current to the focus coil 22 are sufficient to focus thebeam 11 satisfactorily.

In addition to automatically correcting the focus of the beam 11, thepresent invention also allows manual correction for quality of the focusof the beam 11. Accordingly, the output line 97 (see FIGS. 7 and of theNAND gate 94 (see FIG. 10) also supplies the positive output pulses ofthe NAND gate 94 through a line 150 (see FIG. 7) to an eight-twelve bitcounter 151. One suitable example of the eight-twelve bit counter 151 issold by Fairchild Semiconductor as model 9366.

The eight-twelve bit counter 151 counts all of the pulses from theoutput of the NAND gate 94 and obtains an average of the eight envelopesproduced during one cycle of movement of the beam 11 around the path 60by dividing by two or four depending on the size of the beam 11. Bydividing by two or four, the average quality of the focus of the beam 11in both the X and Y axes is obtained.

The output of the eight-twelve bit counter 151 is supplied to aneight-bit digital to analog converter (DAC) 14 152. One suitable exampleof the DAC 152 is sold by Analog Devices Inc. as model DAC SQM.

When the scan of the beam 11 in the +X direction starts as indicated bythe +X SCAN pulse on the line 108 to cause the single shot 107 toproduce a positive pulse, the positive pulse from the single shot 107 isnot only transmitted over the line 106 to the flip flop 102 (see FIG.11) but also is supplied through a line 153 (see FIG. 7) to the eightbit DAC 152 to strobe it. When this occurs, the eight bit DAC 152 storesthe value of the counter 151 until the next strobe and supplies anoutput voltage, which is representative of the average quality of thefocus of the beam 11 in both the X and Y axes at the completion ofacycle of movement of the beam 11 along its entire path 60, on its outputline 154.

A line 155 transmits this signal from the line 154 to a voltmeter 156.The output voltage on the line 154 is proportional to the average widthof the eight envelopes. Through applying the voltage from the eight bitDAC 152 to the voltmeter 156, the quality of the focus supplied to asingle shot 157 to produce a positive pulse on its output line 158. Thepositive pulse on the output line 158 is supplied by a line 159 tomaster reset of the eight-twelve bit counter 151 to reset it to zero.Since this occurs after the eight bit DAC 152 has stored the value ofthe counter 151 and from this supplies the output voltage over the line154, the counter 151 is ready to begin to count again. Since the outputpulse of each of the single shots 107 and 157 is very short, theeighttwelve bit counter 151 is set to zero before the beam 1 1 reachesthe vertical portion 61 (see FIG. 2) of the wire grid 41.

To cause the jitter frequency to be supplied over the line 49 (see FIG.14) when there is not to be automatic adjustment of the quality of thefocus of the beam 11, a positive input voltage must be supplied to theNAND gate 46. This can be supplied from closing a manual switch 160 tosupply a positive input voltage to the OR gate 45. Similarly, a MONITORENABLE signal can be supplied to the OR gate 45 through an opticalisolator 161, which is the same as the optical isolator 44, to the ORgate 45. The input of the MONITOR ENABLE signal from the analog unit 17is over a line 162 to the optical isolator 161. A line 163 from theoptical isolator 161 is grounded.

Thus, by either closing the manual switch 160 or producing the MONITORENABLE signal from the analog unit 17, a positive input voltage issupplied to the NAND gate 46 whereby the jitter frequency appears on theline 49. Of course, it should be understood that the voltmeter 156 alsovisually presents the quality of the beam during the automaticadjustment.

In addition to manual correction for quality of the focus of the beam11, the present invention also allows a determination of whether thebrightness of the beam 11 is sufficient to enable manual correction ofthe focus of the beam 11.

As shown in FIG. 6, an envelope is produced when the focus of the beam11 is satisfactory but there is poor illumination. The envelope 170 hasa non-flat or slanted top. Accordingly, the area beneath the top andabove ninety percent of the peak voltage is smaller, for

example, in comparison with that shown in the envelope 69, which hasgood focus and illumination in FIG. 5. Thus, the number of times thatthe voltage exceeds ninety percent of the peak voltage in the envelope170 is less than in the envelope 69; this indicates that the envelope 69is produced by the beam 11 having better illumination than when theenvelope 170 is produced by the beam 11.

The poor illumination is due to the current density not being the samein all portions of the beam 11. Correction for illumination is obtainedthrough correcting the alignment of the beam 11 as more particularlyshown and described in the copending patent application of Hans C.Pfeiffer et al for Method And Apparatus For Aligning Electron Beams,Ser. No. 393,365, filed Aug. 31, 1973, now U.S. Pat. No. 3,894,271, andassigned to the same assignee as the assignee of this application.

To ascertain whether the illumination of the beam 11 is poor, anillumination detecting means 171 (see FIG. 7) is connected to the outputline 79 of the automatic gain control 76 by a line 172. The line 172 isconnected to a squarer 173 (see FIG. 13), which is a multiplier used asa squarer, of the illumination detecting means 171. The squarer 173makes each of the envelopes produced by the beam 11 passing over thewire grid 41 entirely positive. I

To ascertain each time that the voltage exceeds ninety percent of thepeak voltage as the beam 11 passes over the wire grid 41, the output ofthe squarer 173 is connected to a comparator 174, which produces anoutput each time that its threshold voltage is crossed by the outputvoltage of the squarer 173 exceeding eighty-one percent of the peakvoltage. This is because the squarer 173 reduces the voltage of ninetypercent of the peak voltage to eighty-one percent of the peak voltage.The output of the comparator 174 is connected to an eight-twelve bitcounter 175, which is preferably the same as the eight-twelve bitcounter 151. The counter 175 counts each time that the comparator 174produces an output due to its threshold being crossed.

The eight-twelve bit counter 175 counts all of the pulses from theoutput of the comparator 174 and obtains an average of the eightenvelopes produced during one cycle of movement of the beam 11 aroundthe path 60 (see FIG. 2) by dividing by two or four depending on thesize of the beam 11. By dividing by two or four, the average quality ofillumination of the beam 11 in both the X and Y axes is obtained.

The output of the eight-twelve bit counter 175 (see FIG. 13) is suppliedto an eight bit digital to analog converter (DAC) 176. When the beam 11starts to move in the +X direction as indicated by the +X scan signal onthe line 108 (see FIG. 7) to cause the single shot 107 to produce apositive output pulse, the positive pulse is not only transmitted to theflip flop 102 and the eight bit DAC 152 but also is transmitted by aline 177 to the eight bit DAC 176 (see FIG. 13) to strobe it. When thisoccurs, the eight bit DAC 176 supplies an output voltage, which isrepresentative of the illumination of the beam 11 in both the X and Yaxes at the completion of a cycle of movement of the beam 11 around itsclosed path 60, on its output line 178.

The output voltage on the line 178 is transmitted to a comparator 180.The output voltage on the line 178 is 16 proportional to the averagewidth of the eight envelopes at the ninety percent level of the peakvoltage.

If the output voltage from the eight bit DAC 176 is high enough toindicate that the illumination of the beam 11 is satisfactory, then thethreshold voltage of the comparator 180 is crossed and a positive outputpulse occurs on a line 181. However, if the output voltage from theeight bit DAC 176 is not sufficient to indicate satisfactoryillumination, then the threshold voltage of the comparator 180 is notcrossed and a positive output voltage does not appear on the output line181 from the comparator 180.

The output line 181 of the comparator 180 is connected to an inverter182, which is connected by a line 183 as one input to a NAND gate 184.The other input to the NAND gate 184 is from an inverter 185, whichreceives an input from a comparator 186.

The comparator 186 is connected to the line 154 (see FIG. 7) by a line187 to receive the output voltage of the eight bit DAC 152. Aspreviously mentioned, this voltage is proportional to the average widthof the eight envelopes at the ten percent level of the peak voltage ofthe envelope.

If the voltage from the eight bit DAC 152 exceeds the threshold voltageof the comparator 186 (see FIG. 13), then a pulse appears on an outputline 189, which connects the comparator 186 to the inverter 185. This isindicative of the quality of the focus of the beam 11 being poor.

If the output voltage from the eight bit DAC 152 does not exceed thethreshold voltage of the comparator 186, then there is no output fromthe comparator 186 so that the output line 189 does not have a positivepulse thereon. This indicates that the focus of the beam 11 issatisfactory.

Accordingly, when the focus of the beam 11 is satisfactory, the inverterprovides a positive pulse as the other input to the NAND gate 184.If theillumination is poor, then a positive pulse also appears on the line 183from the inverter 182 because the threshold voltage of the comparator180 was not crossed by the output of the eight bit DAC 176. As a resultof two positive or high signals as the inputs to the NAND gate 184, alow output occurs on output line 190 of the NAND gate 184. An inverter191 changes this signal to a high for supply to a status decoder 192.This high to the status decoder 192 is indicative of poor illuminationand causes the status decoder 192 to produce a signal to illuminate awarning light. As a result, the alignment of the beam 11 must becorrected for the focus and astigmatism of the beam 11 to be manuallycorrected.

The output line 158 (see FIG. 7) of the single shot 157 also isconnected by a line 193 to the master reset of the counter 175 (see FIG.13) to reset it to zero. Since this occurs after the eight bit DAC 176has supplied the output voltage over the line 178, the counter 175 isready to begin to count again at the same time that the counter 151 is.Since the output pulse of each of the single shots 107 and 157 is veryshort, the counter 175 is set to zero before the beam 11 reaches thevertical portion 61 (see FIG. 2) of the wire grid 41 in the same manneras discussed with respect to the counter 151.

Considering the operation of the present invention, the beam 11 isdisposed at the center of its deflection field for manual correction forfocus and astigmatism of the beam 11. Either the MONITOR ENABLE signalis supplied to the optical isolator 161 (see FIG. 14) or the manualswitch 160 is closed to cause a jitter frequency to be supplied to the Xand Y electrostatic deflection plates 3134 (see FIG. 1) in phase fromthe oscillator 47 (see FIG. 14). Manual controls on the column controlunit 16 are turned to adjust the focus of the beam 11 and itsastigmatism in accordance with the voltage on the voltmeter 156. Themanual controls change the resistances of potentiometers in the columncontrol unit 16.

After the beam 11 has been satisfactorily manually adjusted at thecenter of the field. the beam 11 is deflected to each of the corners ofthe field with the focus coil 22 being adjusted initially and then thestigmator coils 21A and 21B being adjusted. Thereafter, the beam 11 isdeflected to the middle of each of the sides of the field and thestigmator coils 21A and 21B are manually adjusted to further correct forastigmatism. This manual adjustment is not necessary for about a yearunless some portion of the electron beam assembly is disassembled. 7

Whenever there is to be automatic adjustment of the focus of the beamduring one of the C cycles, the FOCUS SERVO signal, which is a positivepulse as shown in the timing diagram of FIG. 15, is supplied through theoptical isolator 44 (see FIG. 14) to the OR gate 45 to cause a jitterfrequency to appear on the output line 49 of the dither control 48. Theoutput line 49 supplies this voltage through the analog unit l7 (seeFIG. 1) to the X and Y electrostatic deflection plates 31-34 in phase.

As shown in the timing diagram of FIG. 15, the X magnetic deflectionvoltage to the X magnetic deflection coils 23 and 24 increases from itsminimum to its maximum to move the beam 11 in the +X direction asindicated by the arrow 65 in FIG. 2. The Y magnetic deflection voltageto the Y magnetic deflection coils 25 and 26 starts to decrease from itsmaximum to its minimum as soon as the X magnetic deflection voltagereaches its maximum. Thus, this causes the beam 11 to move in the Ydirection as indicated by the arrow 66 in FIG. 2.

At the time that the FOCUS SERVO signal is supplied, the +X SCAN signal,which is a positive pulse, is supplied from the analog unit 17 (seeFIG. 1) on the line 108 (see FIG. 7). As shown in the timing diagram ofFIG. 15, the +X SCAN pulse remains up for the entire time that the beam11 moves in the +X direction as indicated by the arrow 65 in FIG. 2.

The +X SCAN pulse causes the timing pulse from the single shot 107 (seeFIG. 7) to be supplied through the line 106 to change the state of theflip flop 102 (see FIG. 11). It also Strobes the eight bit DAC 152 (seeFIG. 7) and the eight bit DAC 176 (see FIG. 13) to cause their outputsto be supplied at the same time to the voltmeter 156 (see FIG. 7) andthe comparator 186 (see FIG. 13) from the eight bit DAC 152 and to thecomparator 180 (see FIG. 13) from the eight bit DAC 176. However,illumination is not utilized during the automatic adjustment.Furthermore, the voltmeter 156 is normally not utilized during automaticadjustment of the focus of the beam 11.

It should be understood that the timing pulse from the single shot 107and the timing pulse from the single shot 157 are very short in time incomparison with the +X SCAN as shown in FIG. 15. Thus, the flip flop 102changes state almost as soon as the +X SCAN signal is supplied. 3

As shown in the timing diagram of FIG. 15, the state of the flip flop102 (see FIG. 11) is set so that the counter 110 will count up duringthe deflection of the beam 11 around its path 60 (see FIG. 2). Thus, theNAND gate 101 (see FIG. 11) will supply pulses from the output of theNAND gatee 94 (see FIG. 10) to the counter 110 (see FIG. 11).

As shown in the timing diagram of FIG. 15, the pulse from the output ofthe single shot 112 (see FIG. 11) to the master reset of the counter 110is supplied when the flip flop 102 changes state. Thus, the counter 110is set at zero just after the flip flop 102 changes state to allowpulses from the NAND gate 94 to be supplied through the NAND gate 101 tothe counter 110.

While the beam 11 starts to scan in the +X direction as soon as the +XSCAN signal is supplied to increase the magnetic deflection voltage tothe coils 23 and 24 (see FIG. 1), the flip flop 102 (see FIG. 11) doesnot change state until the negative going portion of the output of thesingle shot 107. When the Q output of the flip flop 102 goes positive,the single shot 112 supplies a positive pulse on the line 113 to resetthe counter 110 to zero. All of this occurs before the beam 11 reachesthe vertical portion 61 (see FIG. 2) of the wire grid 41 during itsmovement in the +X direction.

At the completion of the first deflection cycle of the beam 11 aroundits path 60, another +X SCAN pulse is supplied from theanalog unit 17.The counter 110 has counted each time that the voltage in each of theeight envelopes has exceeded ten percent of the positive or negativepeak voltage. These pulses are stored in the counter 110.

When the +X SCAN pulse occurs to cause the next deflection cycle of thebeam 11 around its closed path 60, the state of the flip flop 102 ischanged by the pulse from the single shot 107 (see FIG. 7) so that the 6output of the flip flop 102 goes positive. This results in the outputpulses from the NAND gate 94 (see FIG. 10) being supplied through theNAND gate 104 (see FIG. 11) to the counter 110. This causes the counter110 to count down from the count stored in the counter 110 during theprior deflection cycle of the beam 11 around its closed path 60.

If the down count exceeds the up count, then a negative pulse appears onthe line 111 as indicated in the timing diagram of FIG. 15 as BORROWWRONG DI- RECTION ON LINE 111. This negative signal occurs during or atthe end of the down count depending on when the down count exceeds theup count. If the down count does not exceed the up count, then no signalappears on the line 111.

When the FOCUS SERVO signal starts, the FOCUS SERVO INITIATE signal (seeFIG. 15) is supplied from the analog unit 17 (see FIG. 1) to the line119 (see FIG. 12) to set the flip flop 118 in the condition in which theQ output of the flip flop 118 is positive. The Q output of the flip flop118 is supplied as one input to each of the NAND gates 116 and 121.

When a positive pulse from the single shot 114 (see FIG. 11) occurs dueto the flip flop 102 changing state so that the Q output goes positiveto start the down count, the NAND gate 121 (see FIG. 12) supplies anegative pulse on the line 122 since both of its inputs are positive.This causes one of the NAND gates 123 and 124, depending on the state ofthe flip flop 125, to have its output supply a positive pulse to thecounter 127 whereby the eight bit DAC 128 supplies a change in voltageto the focus coil driver 130 (see FIG. 7) to change the current to thefocus coil 22.

If the down count in the counter 110 (see FIG. 11) exceeded the upcount, then the negative pulse on the line 111 causes the NAND gate 116(see FIG. 12) to produce a positive pulse on the line 126 to change thestate of the flip flop 125. Accordingly, when the counter 110 completesthe up count during the next cycle of deflection of the beam 11 aroundits closed path 60 (see FIG. 2) over the wire grid 41, the other of theNAND gates 123 (see FIG. 12) and 124 will supply the signal to thecounter 127 to reverse the direction of count in the counter 127 wherebythe eight bit DAC 128 changes the direction in which the magnitude ofthe output voltage is being altered. If this change in voltage resultsin the down count in the counter 110 (see FIG. 11) again exceeding theup count during the 7 next down count cycle, then the flip flop 125 (seeFIG.

12) again changes state. This is shown in FIG. by a second negativepulse on the line 111 (see FIG. 11) no later than completion of the downcount. If at the end of the third down count cycle, another negativepulse appears on the line 111 to indicate that the down count exceededthe up count in the counter 110, then the focus is at its bestadjustment as previously mentioned. As a result, the three state counter135 (see FIG. 12), which has counted each time that there was a positivepulse on the line 126, causes the flip flop l 18 to change state througha negative pulse from the single shot 136 to the R input of the flipflop 118.

The changing of the state of the flip flop 118 prevents any furthernegative pulse from being supplied from the NAND gate 121. It alsoprevents the NAND gate 116 from supplying positive pulses as an outputsince the output of the NAND gate 116 remains up. This is because theinput to the NAND gates 116 and 121 from the O output of the flip flop118 remains negative.

The FOCUS SERVO signal is supplied to the optical isolator 44 (see FIG.14) throughout the C cycle. During the time that the optical isolator 44is supplying an input to the OR gate 45 to activate the dither control48, this signal also is supplied through an inverter 194 and a line 195to the analog unit 17 (see FIG. 1) to inhibit any other operations whichoccur during the C cycle such as correcting for alignment of the beam11, for example, as shown and described in the aforesaid Pfeiffer et alapplication.

While the present invention has shown and described a change in thevoltage to the focus coil driver 130 occurring only after the end of anup count to the counter 110 and a reversal in the direction in which themagnitude is being changed only during the down count or at the endthereof, it should be understood that such is not a requisite of thepresent invention. Thus, if desired, each cycle of deflection of thebeam 1 1 along its closed path 60 could be compared with the prior cycleand correction made to the focus of the beam 11 after each cycle ratherthan after every other cycle. Of course, this would require a differentcircuit arrangement.

While the present invention has shown and described the beam 11 as beingmoved in the square path 60, it should be understood that such is not arequisite for satisfactory operation. Thus, the beam 11 could bedeflected in any closed path such as a circle or a rectangle, forexample. Furthermore, it is not even necessary for the beam 11 to bedeflected or moved in a closed path but only for the beam 11 to movealong the same predetermined path during each cycle of movement.

Anadvantage of this invention is that the focus of a beam of chargedparticles can be automatically adjusted. Another advantage of thisinvention is that the quality of the beam focus is visually obtainable.A further advantage of this invention is that the quality ofillumination of the beam is visually obtainable.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

l. A method for correcting an aberration of a beam of charged particlesincluding:

directing the beam at a constant velocity in a predetermined path acrossa target during each cycle; ascertaining the lengths of time for thebeam to enter and leave at least one portion of the target;

and adjusting the aberration of the beam in accordance with the lengthsof time for the beam to enter and leave the portion of the target incomparison with the lengths of time for the beam to enter and leave theportion 'of the target during the prior cycle of movement of the beam inits predetermined path.

2. The method according to claim 1 including:

oscillating the beam in a single predetermined direction during itsmovement in the predetermined path with the single predetermineddirection being other than any direction of movement of the beam in itspredetermined path;

and ascertaining the length of time for the beam to enter the portion ofthe target by voltage signals produced by the oscillations when the beamenters the portion of the target and the length of time for the beam toleave the portion of the target by voltage signals produced by theoscillations when the beam leaves the portion of the target.

3. The method according to claim 2 including:

directing the beam in a closed path to provide the predetermined path;

and forming the target in the shape of a cross so that four portions ofthe target are crossed during each movement of the beam in the closedpath.

4. The method according to claim 3 including:

counting the number of times that the voltages produced by theoscillations pass a predetermined voltage limit during each entrance orexit of the beam over the portion of the target to ascertain the lengthof time for the beam to enter or leave the portion of the target;

and adjusting the aberration of the beam in accordance with the countduring a cycle of movement of the beam in its predetermined path incomparison with the count during the prior cycle of movement of the beamin its predetermined path.

5. The method according to claim 4 in which:

the aberration of the beam is the quality of the focus of the beam;

and the focus is adjusted by changing the current flow through the focuscoil.

6. The method according to claim 5 including automatically reversing thedirection of change of the current flow through the focus coil if thefocus quality decreases.

7. The method according to claim 5 including:

21 adjusting the focus only every other cycle of movement of the beam inits predetermined path; and adjusting the focus only after completion ofa cycle of movement of the beam in its predeter- 22 means to move thebeam at a constant velocity in a predetermined path across a targetduring each cycle; means to ascertain the lengths of time for the beamto mined path f ll Wiflg C mPlCIi H f th cycl of enter and leave atleast one portion of the target movement Of the beam in itspredetermined path in during each cycle of movement in the predeterwhichcomparison of the count with the count from i d h; the Prior Cycle ofmovement of the beam in its P and means to correct the aberration of thebeam i detemhhed P l0 accordance with the lengths of time for the beamto e method aeeordmgto elalm Zmehldmg? enter and leave the portion ofthe target in compareouhtlhg the humheh of thhes that the Voltages Pison with the lengths of time for the beam to enter dueed y toeelhatlohs P a Predetemhhed and leave the portion of the target duringthe prior voltage limit during each entrance or exit of the cycle fmovement f the beam i its Predetflbeam over the portion of the target toascertain the mined path length of time for the beam to enter or leavethe The System according to Claim 16 including; P e the target; means tooscillate the beam in a single predetermined and adlustmg the aberranonof the b m accor direction during its movement in the predeterdance wlthh F durmgfl cycle movemeht mined path by' said moving means with thesingle of theibeam predtermmeq path m compan' 20 predetermined directionbeing other than any di- Son wlth the count durmg pnor Cycle of moverection of movement of the beam in its predeterment of the beam in itspredetermined path. mined path. 5 method i il i 9 clhalm 8 It F f saidascertaining means including: t f h g t 6 cam is t e qua 0 t e Ocusmeans to obtain the voltages produced by said oso t e cillating meanswhen the beam enters and leaves and the focus lS ad usted by changingthe current i the portion of the target, flow through the focus coil.

and means to count the number of times that the 10. The method accordingto claim 9 including autovoltages produced by the beam entering andmatrcally reversing the direction of change of the curleaving theportion of the target pass a predeterrent flow through the focus COll ifthe focus quality decreases mined voltage limit during each cycle ofmovement of the beam in its predetermined path; 11. The method accordingto claim 9 including:

. and said correcting means includes means to correct ad usting thefocus only every other cycle of movethe aberration of the beam inaccordance with the ment of the beam in its predetermined path;

count obtained by said counting means during a and ad usting the focusonly after completion of a cycle of movement of the beam in itspredetermined path in comparison with the count obtained by saidcounting means during the prior cycle of movement of the beam in itspredetermined path.

18. The system according to claim 17 in which said oscillating meanscomprises means to apply a single high frequency jitter to said movingmeans.

19. The system according to claim 18 in which:

said moving means includes first and second means to deflect the beam inorthogonal directions;

and said first and second deflecting means moves the beam in a closedpath across portions of the target extending in the orthogonaldirections.

20. The system according to claim 19 in which:

the aberration of the beam is the quality of the focus of the beam;

and said correcting means includes means to change the focus of the beamin accordance with the count obtained by said counting means during acycle of movement of the beam in its predetermined path in comparisonwith the count obtained by said counting means during the prior cycle ofmovement of the beam in its predetermined path.

21. The system according to claim 20 in which said focus change meansincludes means to change the focus only every other cycle of movement ofthe beam in its predetermined path with the change occurring only aftercompletion of a cycle of movement of the beam in its predetermined pathfollowing completion of the cycle of movement of the beam in itspredetermined path in which comparison of the count with the count fromthe prior cycle of movement of the beam in its predetermined pathoccurred.

cycle of movement of the beam in its predetermined path followingcompletion of the cycle of movement of the beam in its predeterminedpath in which comparison of the count with the count from the priorcycle of movement of the beam in its pre- 4 determined path occurred.

12. The method according to claim 1 in which:

the aberration of the beam is the quality of the focus of the beam;

and the focus is adjusted by changing the current flow through the focuscoil.

13. The method according to claim 12 including automatically reversingthe direction of change of the current flow through the focus coil ifthe focus quality decreases.

14. The method according to claim 13 including:

directing the beam in a closed path to provide the predetermined path;

and forming the target in the shape of a cross so that four portions ofthe target are crossed during each movement of the beam in the closedpath.

15. The method according to claim 12 including:

adjusting the focus only every other cycle of movement of the beam inits predetermined path;

and adjusting the focus only after completion of a cycle of movement ofthe beam in its predetermined path following completion of the cycle ofmovement of the beam in its predetermined path in which comparison ofthe count with the count from the prior cycle of movement of the beam inits predetermined path occurred.

16. A system for correcting an aberration of a beam of charged particlesincluding:

22. The system according to claim 17 in which said counting means countsthe number of times that the voltages exceed a predetermined minimumvoltage.

23. The system according to claim 22 in which the predetermined minimumvoltage is ten percent of the peak voltage.

24. The system according to claim 23 in which:

the aberration of the beam is the quality of the focus of the beam;

and said correcting means includes means to change the focus of the beamin accordance with the count obtained by said counting means during acycle of movement of the beam in its predetermined path in comparisonwith the count obtained by said counting means during the prior cycle ofmovement of the beam in its predetermined path.

25. The system according to claim 24 in which said focus change meansincludes means to change the focus only every other cycle of movement ofthe beam in its predetermined path with the change occurring only aftercompletion of a cycle of movement of the beam in its predetermined pathfollowing completion of the cycle of movement of the beam in itspredetermined path in which comparison of the count with the 24 countfrom the prior cycle of movement of the beam in its predetermined pathoccurred.

26. The system according to claim 17 in which:

the aberration of the beam is the quality of the focus of the beam;

and said correcting means includes means to change the focus of the beamin accordance with the count obtained by said counting means during acycle of movement ofthe beam in its predetermined path in comparisonwith the count obtained by said counting means during the prior cycle ofmovement of the beam in its predetermined path.

27. The system according to claim 26 in which said focus change meansincludes means to change the focus only every other cycle of movement ofthe beam in its predetermined path with the change occurring only aftercompletion of a cycle of movement of the beam in its predetermined pathfollowing completion of the cycle of movement of the beam in itspredetermined path in which comparison of the count with the count fromthe prior cycle of movement of the beam in its predetermined pathoccurred.

28. The system according to claim 16 in which said oscillating meanscomprises means to apply a single high frequency jitter to said movingmeans.

1. A method for correcting an aberration of a beam of charged particlesincluding: directing the beam at a constant velocity in a predeterminedpath across a target during each cycle; ascertaining the lengths of timefor the beam to enter and leave at least one portion of the target; andadjusting the aberration of the beam in accordance with the lengths oftime for the beam to enter and leave the portion of the target incomparison with the lengths of time for the beam to enter and leave theportion of the target during the prior cycle of movement of the beam inits predetermined path.
 2. The method according to claim 1 including:oscillating the beam in a single predetermined direction during itsmovement in the predetermined path with the single predetermineddirection being other than any direction of movement of the beam in itspredetermined path; and ascertaining the length of time for the beam toenter the portion of the target by voltage signals produced by theoscillations when the beam enters the portion of the target and thelength of time for the beam to leave the portion of the target byvoltage signals produced by the oscillations when the beam leaves theportion of the target.
 3. The method according to claim 2 including:directing the beam in a closed path to provide the predetermined path;and forming the target in the shape of a cross so that four portions ofthe target are crossed during each movement of the beam in the closedpath.
 4. The method according to claim 3 including: counting the numberof times that the voltages produced by the oscillations pass apredetermined voltage limit during each entrance or exit of the beamover the portion of the target to ascertain the length of time for thebeam to enter or leave the portion of the target; and adjusting theaberration of the beam in accordance with the count during a cycle ofmovement of the beam in its predetermined path in comparison with thecount during the prior cycle of movement of the beam in itspredetermined path.
 5. The method according to claim 4 in which: theaberration of the beam is the quality of the focus of the beam; and thefocus is adjusted by changing the current flow through the focus coil.6. The method according to claim 5 including automatically reversing thedirection of change of the current flow through the focus coil if thefocus quality decreases.
 7. The method accorDing to claim 5 including:adjusting the focus only every other cycle of movement of the beam inits predetermined path; and adjusting the focus only after completion ofa cycle of movement of the beam in its predetermined path followingcompletion of the cycle of movement of the beam in its predeterminedpath in which comparison of the count with the count from the priorcycle of movement of the beam in its predetermined path occurred.
 8. Themethod according to claim 2 including: counting the number of times thatthe voltages produced by the oscillations pass a predetermined voltagelimit during each entrance or exit of the beam over the portion of thetarget to ascertain the length of time for the beam to enter or leavethe portion of the target; and adjusting the aberration of the beam inaccordance with the count during a cycle of movement of the beam in itspredetermined path in comparison with the count during the prior cycleof movement of the beam in its predetermined path.
 9. The methodaccording to claim 8 in which: the aberration of the beam is the qualityof the focus of the beam; and the focus is adjusted by changing thecurrent flow through the focus coil.
 10. The method according to claim 9including automatically reversing the direction of change of the currentflow through the focus coil if the focus quality decreases.
 11. Themethod according to claim 9 including: adjusting the focus only everyother cycle of movement of the beam in its predetermined path; andadjusting the focus only after completion of a cycle of movement of thebeam in its predetermined path following completion of the cycle ofmovement of the beam in its predetermined path in which comparison ofthe count with the count from the prior cycle of movement of the beam inits predetermined path occurred.
 12. The method according to claim 1 inwhich: the aberration of the beam is the quality of the focus of thebeam; and the focus is adjusted by changing the current flow through thefocus coil.
 13. The method according to claim 12 including automaticallyreversing the direction of change of the current flow through the focuscoil if the focus quality decreases.
 14. The method according to claim13 including: directing the beam in a closed path to provide thepredetermined path; and forming the target in the shape of a cross sothat four portions of the target are crossed during each movement of thebeam in the closed path.
 15. The method according to claim 12 including:adjusting the focus only every other cycle of movement of the beam inits predetermined path; and adjusting the focus only after completion ofa cycle of movement of the beam in its predetermined path followingcompletion of the cycle of movement of the beam in its predeterminedpath in which comparison of the count with the count from the priorcycle of movement of the beam in its predetermined path occurred.
 16. Asystem for correcting an aberration of a beam of charged particlesincluding: means to move the beam at a constant velocity in apredetermined path across a target during each cycle; means to ascertainthe lengths of time for the beam to enter and leave at least one portionof the target during each cycle of movement in the predetermined path;and means to correct the aberration of the beam in accordance with thelengths of time for the beam to enter and leave the portion of thetarget in comparison with the lengths of time for the beam to enter andleave the portion of the target during the prior cycle of movement ofthe beam in its predetermined path.
 17. The system according to claim 16including: means to oscillate the beam in a single predetermineddirection during its movement in the predetermined path by said movingmeans with the single predetermined direction being other than anydirection of movement of the beam in its predetermined path; saidascertaining means including: means to obtain the voltages produced bysaid oscillating means when the beam enters and leaves the portion ofthe target; and means to count the number of times that the voltagesproduced by the beam entering and leaving the portion of the target passa predetermined voltage limit during each cycle of movement of the beamin its predetermined path; and said correcting means includes means tocorrect the aberration of the beam in accordance with the count obtainedby said counting means during a cycle of movement of the beam in itspredetermined path in comparison with the count obtained by saidcounting means during the prior cycle of movement of the beam in itspredetermined path.
 18. The system according to claim 17 in which saidoscillating means comprises means to apply a single high frequencyjitter to said moving means.
 19. The system according to claim 18 inwhich: said moving means includes first and second means to deflect thebeam in orthogonal directions; and said first and second deflectingmeans moves the beam in a closed path across portions of the targetextending in the orthogonal directions.
 20. The system according toclaim 19 in which: the aberration of the beam is the quality of thefocus of the beam; and said correcting means includes means to changethe focus of the beam in accordance with the count obtained by saidcounting means during a cycle of movement of the beam in itspredetermined path in comparison with the count obtained by saidcounting means during the prior cycle of movement of the beam in itspredetermined path.
 21. The system according to claim 20 in which saidfocus change means includes means to change the focus only every othercycle of movement of the beam in its predetermined path with the changeoccurring only after completion of a cycle of movement of the beam inits predetermined path following completion of the cycle of movement ofthe beam in its predetermined path in which comparison of the count withthe count from the prior cycle of movement of the beam in itspredetermined path occurred.
 22. The system according to claim 17 inwhich said counting means counts the number of times that the voltagesexceed a predetermined minimum voltage.
 23. The system according toclaim 22 in which the predetermined minimum voltage is ten percent ofthe peak voltage.
 24. The system according to claim 23 in which: theaberration of the beam is the quality of the focus of the beam; and saidcorrecting means includes means to change the focus of the beam inaccordance with the count obtained by said counting means during a cycleof movement of the beam in its predetermined path in comparison with thecount obtained by said counting means during the prior cycle of movementof the beam in its predetermined path.
 25. The system according to claim24 in which said focus change means includes means to change the focusonly every other cycle of movement of the beam in its predetermined pathwith the change occurring only after completion of a cycle of movementof the beam in its predetermined path following completion of the cycleof movement of the beam in its predetermined path in which comparison ofthe count with the count from the prior cycle of movement of the beam inits predetermined path occurred.
 26. The system according to claim 17 inwhich: the aberration of the beam is the quality of the focus of thebeam; and said correcting means includes means to change the focus ofthe beam in accordance with the count obtained by said counting meansduring a cycle of movement of the beam in its predetermined path incomparison with the count obtained by said counting means during theprior cycle of movement of the beam in its predetermined path.
 27. Thesystem according to claim 26 in which said focus change means includesmeans to change the focus only every other cycle of movement of the beamin its predetermined path with the change occurring only aftercompletion of a Cycle of movement of the beam in its predetermined pathfollowing completion of the cycle of movement of the beam in itspredetermined path in which comparison of the count with the count fromthe prior cycle of movement of the beam in its predetermined pathoccurred.
 28. The system according to claim 16 in which said oscillatingmeans comprises means to apply a single high frequency jitter to saidmoving means.